In large-capacity power conversion devices, the converter output is high voltage or large current, and therefore, many large-capacity power conversion devices are configured with a plurality of converters multiplexed in series or parallel. Multiplexing converters allows not only increase in the converter capacity but also reduction in harmonics contained in an output voltage waveform by combining outputs. It is known that, as a result, it is possible to reduce harmonic current flowing out to a grid.
There are various methods for multiplexing converters, e.g., reactor multiplexing, transformer multiplexing, and direct multiplexing. In the case of transformer multiplexing, since an AC side is isolated by transformers, there is an advantage that common DC current can be used among the transformers. However, there is a disadvantage that, in the case where output voltage is high, the configuration of the multiplexed transformer is complicated and the cost of the transformer increases.
Considering the above, as a power conversion device that is suitable for high-voltage usage and does not require a multiplexed transformer, a multilevel converter is proposed in which outputs of a plurality of converters are connected in cascade. One example of such multilevel converters is a modular multilevel converter (hereinafter, referred to as an MMC).
The MMC is composed of an arm in which a plurality of unit converters called cells (hereinafter, referred to as converter cells) are connected in cascade. Each converter cell includes a plurality of semiconductor switches and a DC capacitor, and through ON/OFF control of the semiconductor switches, outputs both-end voltage of the DC capacitor and zero voltage.
In the case of three-phase MMC, the arm is formed individually for each phase. The arms for the respective phases are connected in parallel to each other, and their connection terminals at both ends, which are connected in parallel to each other, are used as DC terminals. The arm for each phase is formed from a positive arm and a negative arm each of which has converter cells the number of which is half the total number of converter cells connected in cascade. The connection point between the positive arm and the negative arm is used as an AC-side input/output terminal.
Output of each converter cell of the MMC is connected to both sides of an AC end and a DC end of the MMC. Therefore, each converter cell has a feature of performing both DC output and AC output. That is, current flowing through each arm includes an AC component and a DC component. Therefore, in the MMC, these plurality of current components are controlled. Further, in the MMC, a DC capacitor is provided to each converter cell. Therefore, due to variation among voltages of these DC capacitors, imbalance might occur between voltages of the DC capacitors in the positive arm and voltages of the DC capacitors in the negative arm. Therefore, it is necessary to control voltages of the DC capacitors in order to suppress the imbalance.
The MMC which is a conventional power conversion device includes, as means for controlling voltages of the DC capacitors, command value generating means for generating a circulating current command value on the basis of the voltage values of the DC capacitors in a first arm and the voltage values of the DC capacitors in a second arm.
Further, the conventional MMC includes control means for performing control so that circulating current which is half the sum of current flowing through the first arm and current flowing through the second arm follows the circulating current command value. The command value generating means includes fundamental wave component generating means for generating a fundamental wave component, of the circulating current command value, that has the same phase as terminal voltage between the AC-side input/output terminals, using a difference between a value obtained by averaging the voltage values of all the DC capacitors in the first arm, and a value obtained by averaging the voltage values of all the DC capacitors in the second arm. Further, the command value generating means includes DC component generating means for generating a DC component of the circulating current command value, using a value obtained by averaging the voltage values of all the DC capacitors. Thus, the conventional MMC controls voltages of the DC capacitors by causing the circulating current to flow so as to follow the calculated circulating current command value (see, for example, Patent Document 1).